In cellular mobile radio communication systems, it is beneficial to reduce the transmitter powers of all transmitting mobile radios to the lowest possible level that still allows one or more radio base stations to receive the transmitted signals over noisy radio channels at an acceptable level. For example, Code Division Multiple Access (CDMA) systems employ such mobile transmit power level adjustments. None of the mobile terminals may transmit at a higher level than required to fulfill communication quality demands.
Uplink power control includes an inner loop and an outer loop. An important factor for power control is reliable communication quality measurement, i.e., measuring the quality of the radio communications channel over which the mobile is transmitting. Current systems use, for example, signal-to-interference ratio (SIR) or signal-to-noise ratio to (SNR) to measure channel quality. The inner power control (PC) loop tries to keep the SNR measured for the uplink channel at or close to some target SNR level. The SNR detected for the uplink information is compared with the target, and the mobile is sent power control commands to increase or decrease transmit power to reduce the comparison difference. The outer power controller loop tries to ensure that the errors in the data units received from the mobile radio are at or below an acceptable level. For example, block error rate (BLER) is used in many CDMA systems. The outer PC loop adjusts the inner loop SNR target level based on the uplink channel BLER. If the error rate is too high, the SNR target is increased, and if the error rate is too low, the SNR target is decreased.
The outer PC loop, uplink transmit power control mechanism is normally placed in a radio network controller due to soft handovers where multiple receiving nodes, e.g., base stations, are simultaneously involved in receiving data transmitted by a mobile radio. The inner PC loop is normally placed closer to the mobile radio, e.g., in the base station. As explained below, recent proposals for third generation cellular systems (3G) introducing an uplink retransmission protocol closer to the radio interface complicate the quality measurement information transfer, and therefore, negatively impact power control techniques-based on those measurements.
This problem is explained using FIG. 1 which shows a radio network 10 and a soft handover where two different radio base stations 20a and 20b receive uplink data units from the same transmitting mobile terminal 12. Two decoders 22a and 22b in the two base stations 20a and 20b decode the received data units. The decoded data units are delivered to a diversity combining unit 16 located in the radio network controller 14 to choose or generate from the two sets of data units a single stream of data units. The diversity combining is based on channel quality measurement data provided by the base stations 20a and 20b. In wideband CDMA (WCDMA), the measurement data includes cyclic redundancy checksum indicators (CRCIs) and quality estimates (QEs).
In general, the CRCI indicates the correctness/incorrectness of a data unit, and the QE represents the channel bit error rate (BER). More specifically, the CRCI indicates with very high reliability whether the data unit is erroneous. So a CRCI has only two possible values: the data unit is accurate or the data unit is not accurate. Because there is no ambiguity, the CRCI is an important input for the outer loop power controller 18 and for the diversity combining unit 16 which are both located in the RNC 14. If the diversity combining scheme receives two data units, where one data unit is indicated to be inaccurate and the other one is indicated to be accurate, then the combining scheme delivers the error-free data unit to higher layers and discards the erroneous one. The QE is a real number metric proportional to the experienced channel quality, and it can be used as a relative quality measure between two or more copies of the same data unit. Two data units may have the same CRCI but different QE values. In that case, the diversity-combining unit chooses the data unit with the highest QE, i.e., the data unit with the largest number of correct bits. This kind of diversity selection is useful for applications that can make use of “partly” erroneous/correct blocks, such as vocoders.
During soft handover, the mobile radio transmitter receives power control commands from two independent inner loop power controllers 24a and 24b. Recall that a main objective of the outer power control loop in WCDMA is to adjust the inner power control loop SNR target value so that the estimated block error rate (BLER) agrees with a target BLER value. The outer PC loop increases SNR if the BLER for the communication is too high and decreases the SNR if the BLER is too low. The outer loop power controller 18, located in the radio network controller, assigns the same SNR target for different (independent) inner loop power controllers 24a and 24b used during soft handover. The outer power control loop is driven by processed measurement data provided by the diversity combining unit 16.
A robust error control/correction scheme can be useful in reducing transmitter power levels, and one attractive error control technique is the class of automatic repeat request (ARQ) protocols that can guarantee reliable information transfer over extremely noisy radio channels. In ARQ, the receiver sends retransmission requests of erroneous data units, including missing data units, to the transmitter. Like outer loop power control functionality, ARQ functionality is normally placed in the radio network controller.